The synthesis of high-molecular-weight alcohols (containing between 4 and 8 carbon atoms) from low-molecular-weight alcohols (e.g. methanol and ethanol) has acquired considerable interest in recent years due to the potential use of these oxygenated compounds in the preparation of automotive fuels and also additives for the latter.
In particular, n-butanol (n-ButOH), with a worldwide production of approx. 4-5 million tons per year, is primarily used as an additive for liquid fuels, as a starting chemical reagent in the plastics industry and also as an extraction agent in the aroma and fragrance industry. N-butanol is primarily produced through the petrochemical pathway. Methods for producing n-butanol by fermentation are also widely known.
Alternatively, n-butanol can be obtained by means of the well-known Guerbet reaction, which makes it possible to convert a low-molecular-weight alcohol (e.g. ethanol) into a linear or branched alcohol with a higher molecular weight in the presence of a metal alkoxide dissolved in the alcohol to be transformed. The main disadvantages associated with the Guerbet reaction are: i) the production of water, which must be eliminated from the reaction medium in order to favour the formation of the desired compounds, ii) the production of carboxylic acids, and iii) the use of homogeneous catalysts, which cause both corrosion in the reactors and final separation problems.
U.S. Pat. No. 5,300,695 discloses K, Na, Ba and Cs cations exchanged zeolites, amongst others, as catalysts in the condensation of low-molecular-weight alcohols, to obtain selectivities to iso-butanol of 30%-35% at a 45% methanol conversion rate. Several basic oxides containing Cu, commonly used in the production of high-molecular-weight alcohols from syn-gas (CO/H2), have also been assayed in methanol and ethanol condensation reactions, to produce high-molecular-weight alcohols, albeit with a relatively low production of C4 alcohols (U.S. Pat. No. 5,387,570). Another group of catalysts used are calcium-phosphate-based materials of the hydroxyapatite type (US20070255079). These catalysts have been tested in fixed-bed continuous reactors at 300° C. with very short contact times. The best results were obtained for materials with a Ca/P molar ratio of 1.5-1.7, with ethanol conversion of 12% and selectivities to C4 alcohols (primarily iso-butanol) close to 78%. Oxides of alkaline-earth metals, particularly materials based on MgO with supported transition metals, have also been used for the transformation of ethanol into n-butanol.
In recent years, numerous scientific publications and patents have appeared on the use of hydrotalcite-type materials as catalysts in alcohol condensation reactions, such as the Guerbet reaction, in both batch systems and fixed-bed continuous reactors. The studies performed with these mixed Mg—Al oxides revealed that the catalytic activity of these materials is dependent on the nature, density and strength of the basic surface sites, which, in turn, are dependent on the molar Mg/Al composition in the solid. For example, international application WO2009026510 discloses a process for synthesising n-butanol by means of a material derived from the thermal decomposition of a hydrotalcite which preferably comprises magnesium and aluminum. Moreover, WO2009097312, US20100160693 and WO2009097310 disclose materials obtained by the thermal decomposition of hydrotalcites modified by the inclusion of metal carbonates and ethylenediamine-tetraacetates, which have been developed by DU PONT as catalysts in alcohol condensation reactions operating in a fixed bed at 300° C. and atmospheric pressure. The best results under these conditions have been achieved with a material derived from Mg—Al-based hydrotalcite (containing OH− as the anion), which presents high ethanol conversion (≈44%) with moderate selectivities (≈44%) to n-butanol. When these same materials were assayed in the catalytic conversion of ethanol into n-butanol in the presence of hydrogen in the reaction system, the yields of n-butanol obtained were significantly lower in all cases.
Given the importance of higher alcohols, new, improved catalysts are still needed for the synthesis thereof.